The invention relates to a method for the regulation of the discharge process in a cathode sputtering apparatus which is equipped with current, voltage and pressure regulators for controlling the sputtering process.
In the sputtering of target materials and in the deposition of the sputtered material on substrates, three parameters of decisive importance have a direct influence on the discharge process, that is, the electrical conditions prevailing between cathode and substrate, namely, the voltage, the current and the pressure of the gas present in the discharge zone. This is true with regard to both high-frequency and direct-current sputtering. During the discharge or sputtering process, both the sputtering gas and the parts of the apparatus in the area where the discharge takes place become heated, a fact which is systematically exploited in the glow-discharge treatment of materials, for the purpose of producing high temperatures. In the case of cathode sputtering for the production of thin films, this heating is essentially undesirable, but unavoidable due to the laws of physics.
The cathode sputtering apparatus known hitherto are generally equipped with pressure regulators which are intended for the purpose of keeping the process pressure constant despite the influence of other process parameters. For this purpose, the known apparatus have a pressure sensor whose output is an electrical signal proportional to the pressure, which is delivered together with a reference level signal to the input of the pressure regulator. The output of the pressure regulator is connected to the actuator of a metering valve which is disposed in the gas feed line to the cathode sputtering apparatus. Since gas is continuously being pumped out of the apparatus by vacuum pumps during the cathode sputtering process, it is possible by regulating the gas feed to bring about a state of equilibrium in the apparatus. The gases involved are noble gases, preferably argon, and also noble gases mixed with appropriate reactive gases, such as oxygen, for example, in the case of reactive sputtering. The usual sputtering pressure is on the order of 10.sup.-2 millibars.
In the known cathode sputtering apparatus, furthermore, the discharge current is maintained constant by varying the sputtering voltage accordingly. The sputtering voltage is consequently subject to fluctuations which are necessary for the maintenance of a constant sputtering current. The sputtering power, i.e., the product of current and voltage, is consequently likewise subject to corresponding variations. As stated above, the parts exposed to the plasma and the plasma itself become increasingly hotter during long sputtering operations. In the sealed vacuum chamber, in accordance with the equation of state for gases, the result would be that, as the temperature increases, so does the pressure. Since, however, the pressure is regulated so as to be constant, this means that, as the temperature increases, the weight of the gas in the vacuum diminishes, so that increasingly fewer charge carriers are present in the vacuum chamber. From the electrical viewpoint, the system becomes increasingly resistant, while the current regulator is attempting to keep the discharge current constant by adjusting the voltage. This process reaches its limit at the end of the voltage control range while the discharge power also changes continually.
However, to maintain a uniform build-up of the film, it is highly desirable to maintain a constant discharge or sputtering power throughout the entire sputtering process.